Initial infrastructure for arbitrary precision integer

Initial infrastructure for arbitrary precision integer
codegen support.  This should have no effect on codegen
for other types.  Debatable bits: (1) the use (abuse?)
of a set in SDNode::getValueTypeList; (2) the length of
getTypeToTransformTo, which maybe should be refactored
with a non-inline part for extended value types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43030 91177308-0d34-0410-b5e6-96231b3b80d8